[Heath] Heathkit VTVM model IM-13

[Dick KF4NS]

Thanks Ron, the 12AU7 was replaced with NOS during restoration. You got me thinking  (for me that's 
dangerous hi hi) and I now remember that I tried to find an exact replacement for the 10K pot and could 
not find one of those with 10 turns and the ball bearings design so I am guessing that there was a 
problem with the pot when I tested it, maybe some dead spots, so I took the easy way out using my 
resistor decade box.. I guess it is not worth worrying about since alignment is good now and let's face 
it, the accuracy is never going to be great. Right?

While on topic, I wanted to mention that I made the mod in my IM-18 replacing the battery with a custom 
DC supply. That does not work so great so I decided not to do that in the IM-13. Just have to remember to 
check the battery occasionally so I don't have the mess I  had  in the IM-18.

Thanks for the reply my friend.
73, Dick KF4NS
St Petersburg, FL 33714 USA
Keep The Glow!

----- Original Message ----- 
From: "Ron" <ra.ah at verizon.net>
To: "Dick KF4NS" <kf4nsradio at verizon.net>; <heath at puck.nether.net>
Sent: Friday, January 11, 2013 8:49 AM
Subject: Re: [Heath] Heathkit VTVM model IM-13


|
| The original circuit had a 10K pot for Ohms adjustment. This gave a
| 0 to 10K range. With the added 5.6K, the range became 5.6K to 15.6K.
| The effect of the added resistor was a shift in the adjustment point.
|
| I have built and repaired a large number of Heathkit VTVMs (this
| portion of the circuit was common to most, if not all, models) and
| cannot remember that such a mod was ever necessary. I'll venture a
| guess that, due to component tolerances (say, meter too sensitive
| or an out of balance 12AU7), your VTVM could not attain correct
| Ohms adjustment and you added this resistor to correct the problem.
|
| -- Ron
|
|
|
|
| On 1/10/2013 6:55 PM, Dick KF4NS wrote:
| > I restored this unit about 10 years ago and put it in the closet until now. The thing works great, 
even
| > better than my nice IM-18.
| >
| > Now more to the point. I did something that most of us have probably done. During restoration I added 
a
| > part and have no idea why.
| > R13 (ohms adjust pot)  now has a 5.6K resistor on the wiper terminal. Wish I knew why I did it. Was 
there
| > a mod somebody created?
| >
| > The pot has very little control from end to end but the meter needle is right on for infinity after a
| > good warm-up. Let me know if this is a known mod and why.
| >
| > Thanks and 73, Dick KF4NS
| > St Petersburg, FL 33714 USA
| > Keep The Glow!
| >
| > _______________________________________________
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| > Heath at puck.nether.net
| > https://puck.nether.net/mailman/listinfo/heath
| >
| >
|
|